The popularity of LCD and other pixellated light valve projectors is increasing as a replacement to traditional CRT projection systems because of their smaller size and lighter weight. In the future, it is also expected that pixellated light valve projectors will become less expensive to manufacture than traditional CRT projectors.
One problem inherent in the new crop of LCD and other pixellated light valve projectors is that the number of displayable pixels often does not correspond to the same number of pixels in the original signal source. This can result, for example, when a pixellated panel designed according to one of the common standards (e.g. SVGA having 800.times.600 pixels) is used to display a signal generated according to a different standard (e.g. VGA having 640.times.480 pixels). Prior art traditional CRT projection systems are format flexible with multi-sync operation, due to their analog design. Such systems can display optimized images with any number of pixels per line and variable number of lines per frame.
In order to match signal resolution to a native fixed pixel format of the pixellated panel in an LCD or other pixellated light valve projector, there are three options currently being practiced. The first option is to create a black border around the image by simply projecting the underscanned pixellated display panel. The second option is to electronically "resize" the image to fully utilize the panel area. The third option is to "window" or show only a small portion of the complete picture thereby sacrificing portions of the picture to accommodate the display size.
A fundamental disadvantage of the first option is that the full display surface of the panel and projection screen in underutilized, and the total brightness of the image is reduced. According to the second option, the digital resizing engine is required to "invent" information by computational interpolation to fill blank pixels in the resized image (e.g. expanding 640 pixels onto a line of 800 pixels). The circuitry required is expensive and can result in defocussing (or aliasing) artifacts due to the uncertainties inherent in the interpolation calculations. An example of a "resizing engine" according to the second option, is set forth in U.S. Pat. No. 5,355,328 (Arbeiter et al).
Several prior art patents have issued relating generally to the field of projection systems with image compression and expansion.
U.S. Pat. No. 5,500,692 discloses an image projecting apparatus which uses micro-lenses and mechanical adjustment of the optical axis to eliminate keystoning.
U.S. Pat. Nos. 5,386,252 and 5,499,062 disclose a projection system including means for compressing and expanding the aspect ratio of a picture. These two patents address the problem of projecting a picture whose vertical length is different from its horizontal length, whereby upon projection, the source image generated on the face plate panel of the projection CRT becomes oblong. Picture compression apparatus is disclosed in these two patents for distorting the original image so that the aspect ratio of the picture as applied to the CRT (or LCD panel) becomes one (i.e. it is square). The inventions described in these two patents are said to improve luminance and contrast of the magnified picture projected onto a screen when the original image size has an aspect ratio other than one.
U.S. Pat. No. 5,489,952 discloses a digital scaling device to accommodate different broadcast standards utilizing interpolation means to format data for a number of standardized--format video broadcast signals and to eliminate artifacts.
U.S. Pat. No. 5,483,299 discloses an automatic projection zoom based on measured distance between the projector and the screen. Distance measuring is used to control zoom of the projection lens and is also used to drive a picture quality correction circuit for adjusting luminance, contrast, edge emphasis, etc., in accordance with the distance between the projector and the screen.
U.S. Pat. No. 5,459,532 discloses an automatic focus adjuster for projection display systems wherein a range measuring device is used for measuring the distance between the projection lens and a screen via infrared light, and automatic adjustment elements are used to adjust the focal point of the projection lens system to an optimum focal point or position based on the measured distance. The system functions automatically each time the projection display is moved or repositioned.
U.S. Pat. No. 5,475,447 relates to an automatic digital convergence system including a pattern generator for generating pattern video signals to drive RGB tubes, and a TV camera which takes pictures of the screen. These pictures are digitized via an A/D converter, stored in a frame memory and then processed via a CPU for automatic registration adjustment in relation to each beam.
U.S. Pat. No. 5,200,815 is similar to the '447 patent in that it deals with the problem of automatic convergence correction of projected color images. The invention relates to a system for calculating converging angles of individual projector tubes and automatically determining correction data for deflection linearity, luminance and focusing.
None of the above-discussed prior art patents provide any description of a solution to the problem of optical resizing in pixellated panel projection systems.